Chemical vapor deposition (CVD) is a process whereby a film is deposited on a substrate by reacting chemicals together in the gaseous or vapor phase to form a film. The gases or vapors utilized for CVD are gases or compounds that contain the element to be deposited and that may be induced to react with a substrate or other gas(es) to deposit a film. The CVD reaction may be thermally activated, plasma induced, plasma enhanced or activated by light in photon induced systems.
CVD is used extensively in the semiconductor industry to build up wafers. CVD can also be used for coating larger substrates such as glass and polycarbonate sheets. Plasma enhanced CVD (PECVD), for example, is one of the more promising technologies for creating large photovoltaic sheets, LCD screens, and polycarbonate windows for automobiles.
FIG. 1 illustrates a cut away of a typical PECVD system 100 for large-scale deposition processes—currently up to 2.5 meters wide. This system includes a vacuum chamber 105 of which only two walls are illustrated. The vacuum chamber 105 houses a linear discharge tube 110. The linear discharge tube 110 is formed of an inner conductor 115 that is configured to carry a microwave signal, or other signals, into the vacuum chamber 105. This microwave power radiates outward from the linear discharge antenna 115 and ignites the surrounding support gas 120 that is introduced through the support gas tube 120. This ignited gas is a plasma and is generally adjacent to the linear discharge tube 110. Radicals generated by the plasma and electromagnetic radiation disassociate the feedstock gas(es) 130 introduced through the feedstock gas tube 125 thereby breaking up the feedstock gas to form new molecules. Certain molecules formed during the dissociation process are deposited on the substrate 135. The other molecules formed by the dissociation process are waste and are removed through an exhaust port (not shown)—although these molecules tend to occasionally deposit themselves on the substrate. This dissociation process is extremely sensitive to the amount of power used to generate the plasma.
To coat large substrate surface areas rapidly, a substrate carrier (not shown) moves the substrate 135 through the vacuum chamber 105 at a steady rate, although the substrate 135 could be statically coated in some embodiments. As the substrate 135 moves through the vacuum chamber 105, the dissociation should continue at a steady rate and target molecules from the disassociated feed gas theoretically deposit on the substrate, thereby forming a uniform film on the substrate. But due to a variety of real-world factors, the films formed by this process are not always uniform.
Nonconductive and conductive films deposited utilizing plasma enhanced chemical vapor sources have been achieved with many types of power sources and system configurations. Most of these sources utilize microwaves, radio frequency (RF), high frequency (HF), or very high frequency (VHF) energy to generate the excited plasma species.
Those of skill in the art know that for a given process condition and system configuration of PECVD, it is the average power introduced into the plasma discharge that is a major contributing factor to the density of radicalized plasma species produced. These radicalized plasma species are responsible for disassociating the feedstock gas. For typical PECVD processes, the necessary density of produced radicalized species from the plasma must be greater than that required to fully convert all organic materials. Factors such as consumption in the film deposition processes, plasma decomposition processes of the precursor materials, recombination losses, and pumping losses should be taken into consideration.
Depending upon the power type, configuration and materials utilized, the required power level for producing the necessary density of radicalized plasma species can unduly heat the substrate beyond its physical limits, and possibly render the films and substrate unusable. This primarily occurs in polymer material substrates due to the low melting point of the material.
To reduce the amount of heat loading of the substrate, a method of high power pulsing into the plasma, with off times in between the pulsing, can be used. This method allows the plasma during the short high energy pulses to reach saturation of the radicalized species required for the film deposition process and loss to occur, while reducing the instantaneous and continuous heating of the substrate through the reduction of other forms of electromagnetic radiation.
Film property requirements are achieved by setting the process conditions for deposition, including the power levels, pulsing frequency and duty cycle of the source. To achieve required film properties the structure and structural content of the deposited film must be controlled. The film properties can be controlled by varying the radical species content, (among other important process parameters), and as stated in the above, the radical density is controlled primarily by the average and peak power levels into the plasma discharge.
To achieve several important film properties, and promote adhesion to some types of substrates, the films organic content must be finely controlled, or possibly the contents must be in the form of a gradient across the entire film thickness. Current technology, which enables control of only certain process parameters, cannot achieve this fine control. For example, current technology consists of changing the deposition conditions, usually manually or by multiple sources and chambers with differing process conditions, creating steps in the film stack up to achieve a gradient type stack. Primarily the precursor vapor content, system pressure, and or power level at one or more times is used to develop a stack of layers. These methods are crude at best and do not enable fine control. Accordingly, a new system and method are needed to address this and other problems with the existing technology.